1. Field of the Invention
The present invention generally relates to an optical apparatus and an image forming apparatus comprises the optical apparatus.
2. Discussion of the Background
A related art image forming apparatus such as a copying machine and a printer, for example, is generally provided with an optical device to produce an optical beam and uses it to write image information on a photoconductor. Such a related art image forming apparatus may employ a plastic lens to provide the optical beam to meet recent trends of reducing costs and weights, for example. In addition, the related art image forming apparatus has increasingly penetrated its market and expanded the range of uses and, as a consequence, it needs to satisfy the market demands for a higher accuracy of an image magnification in a variable magnification operation.
The related art image forming apparatus is generally configured to modulate the optical beam based on image data and to deflect the optical beam in a main scanning direction by a deflection mechanism (e.g., a polygon mirror) so as to make the optical beam scan a photoconductor surface through optical elements including an Fθ lens.
However, each of the related art image forming apparatuses may have a different main scanning magnification due to a variation in the optical device and/or properties of optical elements such as the Fθ. lens. A defective operation, for example, a magnification error or a misregistration of a writing start position, may also be generated by a change in a refractive index or a shape of the plastic lens and a change in a scanning position on the photoconductor caused by a variation in an environmental temperature or a variation in a temperature of the apparatus therein. Thereby, an image with a high quality may not be provided. When a related art color image forming apparatus for forming a color image by superimposing a plurality of color images issued, the defective operation, including the magnification error or the misregistration of the writing start position, may be generated in a color basis. Thereby, the main scanning magnification or writing start position for each color may need to be corrected.
JP 2003-279873A proposes to an image forming apparatus that comprises a light source to illuminate in response to image data, a deflector to deflect an optical beam output from the light source into a scanning beam running along a main scanning line across an image forming area in a main scanning direction, first and second beam detectors detecting the scanning beam at positions in front-end and rear-end sides at scanning line in the main scanning direction, a clock generator to generate writing clock signals for controlling an illumination of the light source, a measurement mechanism to measure period during a scanning period between detections of the scanning beam by the first and second beam detectors, a phase corrector to correct a phase of the writing clock signals based on correct amount that is set from measurement result.
JP2003-323085A proposes an image forming apparatus that comprises a plurality of beam detectors detecting the scanning beam at positions at scanning line in the main scanning direction, a clock generator to generate writing clock signals for controlling an illumination of the light source, a measurement mechanism to measure a count number of the writing clock signals generated during a scanning period between detections of the scanning beam by one beam detector and another beam detector by predetermined clock number (two position measurement), a main scanning magnification corrector to correct a main scanning magnification by the measurement mechanism and predetermined clock number.
But if an image forming apparatus uses the above phase correction technique and two position measurements for correcting a main scanning magnification, the image forming apparatus cannot correct a main scanning magnification strictly. This is because phase correction coverage is between front-end synchronous detecting position and rear-end image position. So the image forming apparatus cannot apply phase correction technique to between rear-end image position and rear-end synchronous detecting position, and the image forming apparatus has to use two different frequencies of writing clock signals. And the image forming apparatus counts writing clock signals between front-end synchronous detecting position and rear-end synchronous detecting position for correcting a main scanning magnification. So the frequency of writing clock signal changes while the image forming apparatus counts writing clock signals, the image forming apparatus cannot correct a main scanning magnification strictly.